Cardiac output is an important hemodynamic variable widely used in the field of cardiology for diagnosis and other characterizations of the cardiovascular system. It is routinely used as a diagnostic measure of cardiac function, particularly in the diagnosis of congestive heart failure and related cardiac disease. Characterizations of the cardiovasculature using the Windkessel model, employed to obtain measures of vascular compliance, also require a measure of cardiac output.
Thermodilution and dye dilution procedures are considered two of the most accurate measures of cardiac output. They are, however, surgically invasive procedures which require catheterization of the subject. As such, these procedures are inconvenient, time consuming and expensive, as well as undesirable from the patient's perspective. Moreover, notwithstanding the fact that they are considered the most accurate measures of cardiac output, they have a reproducibility of only between ten to twenty percent. Other measures of cardiac output include: "eyeball" estimates, made on the basis of the patients size and subject to considerable error; Doppler measurement techniques which measure blood velocity and calculate output based on a further measure of aorta cross section, and which are time consuming and subject to errors in aorta cross section measurement; and rebreathing techniques, which are considered difficult to implement, and subject to considerable error if not executed with great care. The Minnesota Thoracic Impedance method, a non-invasive measure of cardiac output, is another alternative measure. While this method may be adequate for detecting changes in cardiac output, it is poor at measuring the absolute value of cardiac output.
Others have devised procedures for measuring cardiac output based on measurements of body surface area, age, heart rate, weight, height and ejection time, whereby a quick, non-invasive measure of cardiac output can be obtained. For example, Smulyan et al., in "An Evaluation of the Cardiac Index," disclose a method for measuring cardiac output using a subject's height and weight (factors closely related to body surface area); Krovetz and Goldbloom, in "Normal Standards for Cardiovascular Data--I. Examination of the Validity of Cardiac Index," disclose a method using age, height, weight and heart rate; and Weissler et al., in "Relationships Between Left Ventricular Ejection Time, Stroke Volume, and Heart Rate in Normal Individuals and Patients with Cardiovascular Disease," establish some correlation between ejection time and cardiac output. While these procedures are relatively fast and easy compared to others, and thus are indicated over the slower, more cumbersome and invasive procedures, they are generally less accurate measures of cardiac output. The present invention provides a measure of cardiac output, in the genre of these procedures, with acceptable accuracy for many applications.